The present invention relates to a hydraulic pressure control apparatus for an automatic transmission-equipped vehicle which carries out idling stop.
An automatic transmission includes a plurality of frictional engagement devices such as a clutch and a brake (hereinafter referred to merely as a clutch) in a gear train. The respective frictional engagement devices include friction plates and are operated to come into engagement and release by hydraulic pressure which is applied to the friction plates. The automatic transmission realizes a plurality of speeds (or gear stages) by combination of the engagement and release of the frictional engagement devices. For instance, the automatic transmission includes a low clutch (LOW/C) and a high clutch (HIGH/C), and establishes forward first speed by engagement of the low clutch and third speed by combination of release of the low clutch and engagement of the high clutch.
The respective clutches include a hydraulic pressure chamber and a piston which is movable by a hydraulic pressure which is supplied to the hydraulic pressure chamber. An operating process of the respective clutches includes a stroke reducing process during which the piston runs to abut against the friction plates in an idle stroke of the piston, and an engagement control process during which the hydraulic pressure to be supplied to the hydraulic pressure chamber varies and is controlled to reach a maximum engagement hydraulic pressure in order to allow the piston to actually abut against the friction plates and smoothly shift an engagement state of the friction plates from the slippage state to the full-engagement state.
There has been conventionally proposed a vehicle equipped with such an automatic transmission as described above, which has an idling stop function in order to enhance fuel economy and reduce emission of exhaust gas. That is, in a case where the vehicle is temporarily stopped for waiting at a traffic light at an intersection during travelling of the vehicle or for waiting for passage of a train at a railroad crossing, the engine is allowed to automatically stop, and after that, at a time in which a predetermined condition is satisfied, the engine is restarted to run the vehicle.
Meanwhile, in the automatic transmission, the above-described clutch is brought into an engagement state and a release state by using the hydraulic pressure to be supplied by a mechanically operated oil pump which is driven by engine power. However, an operation of the oil pump is also stopped during an idling stop of the engine, and therefore, a necessary hydraulic pressure cannot be obtained immediately after the engine is restarted.
Accordingly, upon restarting the engine after the idling stop, the stroke reducing process cannot be quickly accomplished to thereby fail to obtain a desired response ability.
In order to solve the above problem, it is considered that supply of the hydraulic pressure is continued using an auxiliary electric oil pump even during the idling stop. However, in such a case, an increase in cost will be caused.
United State Patent Application Publication No. US2008/0060862 A1 discloses a hydraulic pressure control apparatus in which an accumulator having a spring-biased piston inside thereof is connected to a hydraulic circuit via a cut valve to thereby keep the accumulator in a hydraulic storage state. Upon restarting the engine, the cut valve is opened to rapidly discharge the hydraulic pressure pressurized by the spring from the accumulator, so that a lack of the hydraulic pressure supplied by the oil pump is compensated.